Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor which is generally located inside the hub or below the discs. One type of motor in common use is known as a spindle motor. Such motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally result in physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or raceway and ball, in spite of the lubrication layer, in spite of the lubrication layer provided by bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment and resulting requirements in shock resistance. Shocks create relative acceleration between the discs and the drive casting which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, hydrodynamic bearings are being adopted. In these types of systems, lubricating fluid—either gas or liquid—functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relatively high viscosity of oil allows for larger bearing gaps and therefore greater tolerances to achieve similar dynamic performance.
An essential feature of such fluid dynamic bearings is to seal the bearing from the surrounding atmosphere, especially when the bearing or the motor in which the bearing is incorporated is to be used in a disc drive.
In the prior art, especially in designs incorporating a shaft and thrust plate, where the thrust plate faces a counterplate and defines a thrust bearing therewith, the counterplate rests within a recess in the sleeve. To prevent the fluid which is used to support the thrust plate and counterplate for relative rotation from seeping out of this region between the counterplate and sleeve, a recess has typically been defined in the sleeve, and an o-ring placed therein, sealing the gap between the stationary sleeve and the facing, stationary counterplate. However, with time the o-ring loses some of its elasticity, allowing fluid to potentially seep past.
Further, the elastic o-ring of the prior art tends to absorb oil over time, thereby reducing the amount of fluid in the fluid bearing. Such o-rings also tend to outgas, a very undesirable feature in a sealed atmosphere such as is typically found in a disc drive. A further effort has been made to seal the fluid dynamic bearing from the surrounding atmosphere by welding the outer edge of the counterplate to the surrounding sleeve. However, this approach can create stresses on the counterplate and requires expensive equipment. Therefore, an alternative, reliable, inexpensive and easy to assembly approach to sealing the hydrodynamic bearing from the surrounding disc drive atmosphere continues to be sought.